1. Field of the Invention
This invention relates to a method and apparatus for improving the efficiency of a combustion process, and in particular, to a method and apparatus for controlling the combustion process based upon the level of carbon monoxide, opacity and/or unburned hydrocarbons in the exhaust gases.
2. Prior Art
Industrial and commercial facilities throughout the world utilize fossil fuel combustion processes to generate heat. The heat may be used in many different ways, for example, in drying, power generation, space heating, thermal processing of materials, etc. One known use of fossil fuels is to fire boilers for the generation of steam. Boilers produce steam by mixing fuel with air and burning the mixture in a combustion chamber. The heat generated is recovered by passing water through tubes in the boiler to generate steam. Burning the fuel-air mixture in effect combines oxygen in the air with the hydrogen and carbon in the fuel to form water vapor, carbon monoxide, carbon dioxide, and other products.
The efficiency of energy recovery when fossil fuels are burned in steam boilers typically ranges from 70 to 80%, depending upon the characteristics of the fuel burned, the condition of the boiler, and other variables. Most of the lost energy is heat generated by the combustion process which heat is not transferred to the steam but instead raises the temperature of the exhaust gases vented to the atmosphere by a smokestack.
Almost all equipment within which combustion processes are carried out is operated with "excess" air, that is, an amount of air that supplies more oxygen than is theoretically required to burn completely all of the hydrogen and carbon in the hydrocarbon fuel. Supplying a combustion process with excess air prevents loss of unburned fuel in the smokestack, and prevents potentially explosive mixtures of fuel and air. Because air contains only about 20% oxygen, the heating of the remaining 80% of the air, primarily nitrogen, from ambient temperature to the stack exhaust temperature is a major energy loss. Each percent of energy savings by reducing the amount of air heated to the typically 400.degree.-600.degree. F. stack exhaust temperature will save a percent of fuel, hence the desirability of minimizing the amount of excess air.
The traditional technique for determining and controlling the amount of excess air introduced with fuel to a combustion process has been to measure and control the amount of excess oxygen present in the smokestack exhaust gases. The basis for this practice has been the fundamentally correct understanding that the presence of oxygen in the exhaust gases indicates that more than an adequate supply of oxygen has been supplied for the fuel being burned. It is known, however, that the amount of excess air required to burn hydrocarbon fuel completely varies widely, from about 5 to 60 percent, depending upon the type and quality of the fuel burned, the condition of the boiler, and the load on the boiler. The wide variation in excess oxygen required mandates operation of a boiler at the high end of that particular boiler's range of excess air to avoid even occasional operation in a hazardous condition or in violation of pollution control requirements.
In the burning of hydrocarbons, hydrogen atoms are first split from the hydrocarbon molecule and then oxidized to form water vapor. Next, carbon atoms are oxidized to create carbon monoxide, and the carbon monoxide oxidized to create carbon dioxide. It is known that the level of carbon monoxide in the exhaust gas from a combustion process provides a measure of the completeness of combustion. It is also known that carbon monoxide levels, as a function of excess air, decrease rapidly up to a minimum level of excess air, and then remain relatively constant as additional excess air is supplied.
Unfortunately, control of a combustion process cannot be predicated solely upon the quantity of carbon monoxide present because other limitations may require a higher level of excess air than the carbon monoxide level alone. For example in oil or coal-fired boilers, the boiler may begin to smoke before the desired minimum carbon monoxide level is reached. Similarly, other variables, such as the presence of hydrocarbons in the exhaust gases or the temperature of the exhaust gases may limit the amount of excess air necessary for the combustion process. Therefore, control of a combustion process cannot be based upon a single variable such as carbon monoxide, but must take into account other potentially limiting variables.
Prior art devices which have attempted to rely upon more than one variable in controlling a combustion process have suffered from a number of disadvantages. In some devices, undesirable oscillations in control inputs occur. These oscillations may create thermal stresses which can damage the boiler or other vessel in which the combustion is occurring. In yet other systems existing control elements such as fuel flow valves, fan dampers, and feed water valves, must be replaced with electronically controllable servomechanisms, thereby undesirably increasing the costs of such an installation.
Examples of prior combustion control devices and methods which rely at least partially upon measuring the level of carbon monoxide in the exhaust gases include U.S. Pat. No. 3,723,047 issued to Baudelet de Livois. That patent discloses a control network for a combustion process in which properties of the exhaust gases are sensed and used to control the combustion process.